1. Field of the Invention
The invention relates to movement simulators controlled in position, in speed or in acceleration.
2. Description of Related Art
The movement simulators are used, among other things, for testing the frequency response of a component laid on the table top of a movement simulator capable of generating movements such as instantaneous rotational movements around an axis. A particularly interesting category of movements for conducting tests is formed of movements such as the position, the speed or the angular acceleration of the axis is a sinusoidal function characterised by an amplitude and a frequency. A reference signal of the form Aej·w·t is hence applied while adopting a complex notation) and the characteristic measurements are performed on the sensor to be tested. But this test is only valid if the movement simulator has effectively a movement which follows the reference signal. Consequently the actual movement of the movement simulator should be as close as possible with respect to the reference. Incidentally, the word component defines here any inertial component or any type of sensor measuring a position, a speed or an acceleration.
Control systems for movement simulator are already known by the patent application WO2006/131664A or the article <<Implementation of RST controllers for a flexible servo considering practical limitations>> by CHAMPENOIS G. and AP. in Industrial Automation and Control, 1995, p. 209-213 ISBN 0-7803-2081-6.
The structure of the control loop in position (in speed or in acceleration) of a movement simulator is generally complying with the diagram represented on FIG. 1. The physical system to be controlled 10 is formed of a current amplifier 11, of a direct current or alternating current motor 12 (for instance a brushless alternating current motor: <<AC brushless>>), of the axis 13 of the machine and of a sensor 14. The current amplifier 11 receives for instance a signal in the form of a voltage u(t) and applies consequently the intensity i(t) corresponding at the terminals of the electric motor 12. The axis 13 of the movement simulator coupled to the rotor of the electric motor 11, the circulation of a current i(t) adapted in the stator sets in rotation the axis 13 around its axis of symmetry. The sensor 14 measures an instantaneous kinematic magnitude y(t) relative to the movement of the axis 13 of the movement simulator. This measured magnitude y(t) may be either the position of the axis or the speed thereof, or still the acceleration thereof (angular or linear measured magnitude).
The control loop consists of a control law 20 which, from the inputs formed simultaneously of the reference signal c(t) and of the measured magnitude y(t), determines the value of the signal u(t) to apply to the controlled system 10. This command law is established by a synthesis algorithm from a physical modelling of the behaviour of the system to be controlled.
The closed loop presented on FIG. 1 has the particularity of exhibiting a “low-pass” behaviour between the reference c(t) and the measurement y(t). Which means that the quality of the tracking of a sinusoidal reference depends on the frequency of this reference. For the low frequencies (for instance of the order of the Hertz), there is no particular difficulty for ensuring a tracking of the reference without the occurrence of a significant error, so called the tracking error, between the reference c(t) and the measured magnitude y(t), for instance the position. Conversely, the more the frequency of the sinusoidal reference increases, the more the tracking error becomes significant. For these high frequencies, the position of the axis stills follows a sinusoidal movement but with a certain attenuation of the amplitude which is increasingly marked as the frequency rises. The cut-off frequency of the control loop is defined as the frequency for which the amplitude of the reference undergoes a ±3 dB attenuation.
It is desirable that the cut-off frequency, which is an indicator of the performances of the control loop, is as high as possible. Nevertheless the maximal value of the cut-off frequency is limited because of the high frequency dynamics which cannot be taken into account in the modelling of the controlled machine, a modelling on which the synthesis of the control law is based. These high frequency dynamics on the modelling are for instance due to the electric dynamics of the motor or, still, the resonances of the mechanical structure.
Besides, so that the control loop is stable regardless of the movement one wished to impart to the movement simulator, a so-called robust controller should be designed relative to these high frequency dynamics. As a reminder, the robustness property of a control loop guarantees the stability of the control loop when the system to be controlled departs from the rated model. However, from a theoretical viewpoint, robustness and performances are proved to be two antagonistic notions. I.e. the robustness necessary in the control loop provides limitations for the performances of this control law, and in particular lowers the cut-off frequency of this control law. All the known control systems used on the current movement simulators are subject to such constraint.
Moreover, it should be added that the sinusoidal movement of the axis is not only attenuated relative to the reference signal but also phase-shifted relative thereto. This phase-shift rises significantly when the frequency increases. The phase-shift starts to be significant at frequencies much lower than the cut-off frequency of the slaving.
The movement simulators being machines designed for metrology, this attenuation and this phase-shift cause great difficulties for accurate characterisation of the components to be tested.
The purpose of the invention is hence to remedy the shortcomings aforementioned while suppressing or at least while reducing highly any attenuation and any phase-shift of the measured magnitude (position, speed or acceleration) relative to the sinusoidal reference.